1. Field of the Invention
The present invention relates to a photo detector.
2. Description of the Prior Art
Flat panel displays are widely employed in various consumer products such as liquid crystal displays, liquid crystal televisions, plasma televisions, cellular phones, personal digital assistants, digital cameras, display panel of portable game machines, and touch panel of automatic transaction machines. Hence, how to increase the color and brightness of the flat panel display has become an important task. Most of today's displays include photo detectors for detecting ambient lights. By having color and brightness of the display would adjust according to ambient lights, consumers would perceive a much better visual experience.
Referring to FIG. 1, FIG. 1 illustrates a structural view of a photo detector composed of PIN diode according to the prior art. As shown in FIG. 1, a substrate 12 is first provided, and a semiconductor layer 14 composed of polysilicon is formed on the substrate 12. The semiconductor layer 14 preferably has a first doping region 16, a second doping region 18, and an intrinsic region 20 positioned between the first doping region 16 and the second doping region 18. Specifically, the first doping region 16 is doped with p-type dopants and the second doping region 18 is doped with n-type dopants.
An insulating layer 22 is then deposited on the semiconductor layer 14, and an interlayer dielectric 24 is disposed on the insulating layer 22 thereafter. Typically, the insulating layer 22 is composed of silicon nitride having a thickness of about 100 angstroms and the interlayer dielectric 24 is composed of silicon oxide having a thickness of about 3,000 angstroms.
An etching process is then conducted to form two holes 26 within the interlayer dielectric 24 and the insulating layer 22, in which the holes 26 expose a portion of the first doping region 16 and a portion of the second doping region 18. Two electrodes 28 are then formed on the interlayer dielectric 24 and are connected to the first doping region 16 and the second doping region 18 through the holes 26, thus complete the fabrication of a PIN photo detector.
Typically, the intrinsic region of a PIN diode is a high resistance region which un-doped with any movable carriers, and the fabrication of the PIN diode can be achieved along with a standard TFT-LCD fabrication process. However, it should be noted that after the formation of the gate pattern, the gate is used as a mask for conducting a doping process by injecting dopants into the semiconductor layer 14 to form doped region, such as lightly drains or highly drains. While this step is conducted, the region above the intrinsic region is unprotected and n-type dopants such as phosphorus used in the doping process would be injected into the intrinsic region. The result of injecting n-type dopants into the intrinsic region not only reduces the sensitivity of the photo detector but also affects the performance of the device.